The invention relates to high voltage switches which need to pass high negative voltages and have high current carrying requirements. The high voltage switch consists of a wide transistor with a high drive capability and a gate control circuit for providing the gate control for the switching transistor. The high voltage switches are used in many applications both in non-volatile as well as other semiconductor circuits working at high voltage levels.
FIG. 1 illustrates a circuit diagram of a conventional high voltage switch 100. The switching arrangement includes a switching transistor 122 and a gate voltage control circuit implemented using a conventional level shifter 126. An input signal ‘IN’ having a first voltage level and this is converted to an output signal having a second voltage level to driving the transistor 122. The first voltage level can have a voltage range from 0 to VDD. The second voltage level can have a voltage range from VNEG to VDD.
The voltage at output nodes OUTH, OUTHN, OUT1 and OUT1N with different voltages at an input is summarized in TABLE 1.
TABLE 1INPUTOUTPUTINOUTHOUTHNOUT1OUT1N0VNEGVDDVNEG−VtVDDVDDVNEG−VtVNEG
As illustrated in TABLE 1, the potential range for the output node OUTH can vary from VDD to VNEG, whereas the node OUTHN is an inversion of the node OUTH. When the input signal IN changes from a LOW level (0V) to a HIGH level (VDD), then the node OUTH changes voltage levels from VNEG to VDD and during this transition phase, the transistor 112 gets stressed due to a very high Vds across it. This voltage stress is so high that the transistor 112 can go beyond the range of safe operating area. Further, due to high current drive requirement on the transistor 122, it is of high ‘w’ and loads the transistor 112 with a high gate capacitance, thereby increasing a load at the node OUTH makes the node OUTH slower, which increases the time during which the transistor 112 experiences the voltage stress. So if the time during which the transistor 112 experiences the voltage stress is more, it can cause a breakdown of the transistor 112 and other reliability issues. Similarly if the node OUTHN is driven by a high capacitive load, then the transistors 110 and 112 can go into a breakdown state.
The conventional circuit during transition phase has some stress related problems across its output transistors which causes reliability issues during the working.
Therefore, there is a need for a high voltage switching module that provides reduced voltage stress at its driver output.